Known methods of three-dimensional (3D) condensation to form SiGe fins with high percentage germanium do not allow for formation of these fins with a tight fin pitch, such as, for example, a fin pitch of 27 nm for 7 nm technology. For example, known methods of 3D condensation to produce high percentage germanium SiGe fins result in too much lateral reduction in the sizes of fins, and there is not sufficient substrate space to print fins that are thick enough prior to condensation to result in tight fin pitches. In addition, fin shapes are undesirably altered during 3D condensation of relatively tall fins.
Critical thickness limitations also prevent an overall starting thickness for an initial fin before condensation from being too thick, and prevent growing of high percentage germanium SiGe fins as a thick block and cutting of the fins at a tight pitch out of the block. A critical thickness is the thickness at which strain energy (e.g., compressive strain) becomes too large to maintain local equilibrium between mismatched lattice structures of two materials (e.g., silicon layer and SiGe on the silicon layer). Once the critical thickness is reached or is exceeded, the strain is relaxed through misfit dislocation formation. For example, strain in a SiGe layer on silicon will be relaxed upon reaching the critical thickness. The relaxed unstrained state of SiGe (e.g., a resulting SiGe fin) degrades performance of a field-effect transistor (FET), which can benefit from compressive strain. As an example, a critical thickness of 85 atomic percent germanium SiGe is less than 15 nm. A starting thickness of a fin to produce, for example 85 atomic percent germanium SiGe would have to be greater than 15 nm to produce fins at a desired tight pitch, and a desired height. Moreover, subsequent cutting of a block can also relax compressive strain near a location where a cut is made.